--- transmission-2.83.org/libtransmission/transmission.h
+++ transmission-2.83/libtransmission/transmission.h
@@ -28,6 +28,7 @@
 
 #include <inttypes.h> /* uintN_t */
 #include <time.h> /* time_t */
+#include <unistd.h> /* off_t */
 
 #ifdef WIN32
  #define __USE_MINGW_ANSI_STDIO 1
